1. Field of the Invention
The present invention relates to orthopedic fixation devices for use with modular joint implants and more particularly relates to an improved compression screw apparatus for retaining a joint endoprosthesis in a bone wherein the screw is held in position by a small compressing load, and applied by a part of the implant assembly. The compression load prevents a screw backing out as the prosthesis sinks into the bone (subsides). The screw is particularly suitable for use with tibial prostheses.
2. General Background
Fixation screws and nails used in modular joint replacement can sometimes "back out" of proper operative position if the implant subsides. Revision surgery can be required to reassemble and/or replace all or part of the implant device. Several prior art devices used to treat fractures of the hip are discussed and illustrated in an article entitled "Hip Nails For All Occasions" by Raymond G. Tronzo, in the Orthopedic Clinics of North America (Vol. 5, No. 3, July 1974).
A number of bone screws are commercially available or the subject of publications which are retained in position by means of nuts, threads, pins, retaining rings, welds or lock washers. These retainers physically secure the screw in position. Certain disadvantages are present in known mechanical screw retainers if used for fixation of a modular joint replacement. Such mechanical retainers would be subject to fatigue loads, which have a high likelihood of causing failure of the mechanical retention used, resulting in a disassembly of the implant device.
Another disadvantage of prior fixation systems is that in most cases they require additional surgical time for use and are subject to error. The potential negative effects of prolonged surgery include numerous complications, and errors that may lead to failure which injures the patient, requiring additional surgery.
Several orthopedic fixation devices have been patented. The Asnis system shown in U.S. Pat. Nos. 4,383,527 and 4,450,835 uses one or more rigid lag screws which are installed across a fracture site over guide pins to rigidly secure the bone fragments in close proximity to one another.
Other devices for the fixation of fractures have been patented. A variable length fixation device for insertion into a hole formed in two or more bone fragments includes a barrel portion and a fastener element. The barrel portion is secured to an inside surface of the hole in a proximal one of the bone fragments. The fastener element is telescopically mounted to the barrel portion and is extendable into the distal of one of the bone fragments. The assembly prevents lateral movements of the distal fragments, relative to the proximate fragment, while allowing axial relative movements (i.e., linear movements along the longitudinal axis of the fastener element) to occur. In one embodiment, the fastener element is a cannulated bone screw having a hexagonal shaped cross section on one end which is telescopically received into a hex shape interior of the hollow barrel portion. Two components are preferably permanently joined to form a one-piece assembly having a variable overall length. The fixation device is substantially insertable in its entirety into a hole which extends across the fracture site.
Other patents that relate generally to compression screws include U.S. Pat. No. 4,776,329, entitled "Resorbable Compression Screw And Method", wherein a method for repairing a bone fracture with a compression screw assembly is provided. First and second non-resorbable compression members are positioned so that the head portion of the compression screw can protrude from the surface of the second non-resorbable compression member after further compression is effected by the normal healing process. At least the head portion of the screw is formed of a material that resorbs upon contact with body fluids.
In U.S. Pat. No. 4,530,355, there is provided a compression screw assembly for applying compression to a fracture bone. The apparatus includes a lag screw, a compression plate including a hollow barrel member adapted to receive the lag screw in at least one fixed orientation, a wrench assembly adapted to releasably engage the lag screw in axial alignment therewith, and apparatus having surface contours complimentary with the outer surface of the lag screw and inner surface of the barrel member for being optionally insertable into the barrel member to prevent axial rotation of the lag screw with respect to the barrel member.
Another compression screw system is the subject of U.S. Pat. No. 4,095,591, wherein an extension is provided for being non-rotatably fixed to a lag screw that is to be anchored to the head of a femur or other bone in a manner so as to allow compression to be applied to the fracture. The extension extends outward of the bone when attached to the lag screw and when the lag screw is anchored to the bone to allow a compression plate to be easily positioned thereon, the cross section of the extension is substantially the same as the cross section of the lag screw to allow the compression plate to be easily and quickly passed onto the lag screw from the extension once the compression plate has been positioned on the extension.
Other fixating devices which uses bone screws include U.S. Pat. No. 4,432,358, entitled "Compression Hip Screw Apparatus"; U.S. Pat. No. 4,530,355, entitled "Compression Screw Assembly"; U.S. Pat. No. 4,612,920, entitled "Compression Hip Screw"; U.S. Pat. No. 4,621,629, entitled "Compression Hip Screw"; U.S. Pat. No. 4,628,923, entitled "Axial Compression Device"; U.S. Pat. No. 4,653,489, entitled "Fenestrated Hip Screw And Method Of Augmented Fixation"; U.S. Pat. No. 4,657,001, entitled "Antirotational Hip Screw"; U.S. Pat. No. 4,791,918, entitled "Femoral-Neck Implant"; U.S. Pat. No. 4,794,919, "Fixating Device"; and U.S Pat. No. 4,858,601, entitled "Adjustable Compression Bone Screw".